1. Field of the Invention
This invention relates to a new and useful non-yellowing polyurethane coating composition which does not contain any solvent and which has a high solids content. More specifically, it relates to a polyurethane coating composition comprising an aliphatic or alicyclic polyisocyanate containing at least 3 isocyanate groups, a monohydric alcohol in an amount not less than 1/3 of the equivalent weight of the isocyanate groups, and a low-molecular-weight polyhydric alcohol having at least 3 active hydrogen atoms.
2. Description of the Prior Art
Polyurethane coating compositions which comprise aliphatic or alicyclic polyisocyanates containing at least 3 isocyanate groups, high-molecular-weight polyhydric alcohols containing at least 2 hydroxy groups and volatile solvents which do not react with the isocyanates are known in the prior art. Well-known typical examples of the aliphatic or alicyclic polyisocyanates containing at least 3 isocyanate groups include addition reaction products formed between hexamethylene diisocyanate or cyclohexane 1,4-diisocyanate and water or trimethylol propane. Examples of the high-molecular-weight polyhydric alcohols containing at least 2 hydroxy groups are polyethylene glycol, polypropylene glycols, or acrylic polyols or polyester polyols derived from esters formed between these glycols and dicarboxylic acids. The solvent which does not react with the isocyanates may, for example, be a volatile low-viscosity solvent such as benzene, xylene, ethyl acetate, methyl ethyl ketone or ethylene glycol monoethyl ether acetate.
It is well known that conventional polyurethane coating compositions obtained using these components have ideal characteristics with regard to weatherability, durability, intimate adhesion to a substrate, flexural resistance, abrasion resistance and soil resistance when used as paints. It has been thought, however, that in order to attain these characteristics, the polyhydric alcohol to be used must have as high a molecular weight as possible or otherwise, the degree of cross-linking of the cross-linked coating composition would become excessively high deteriorating its flexural resistance and adhesive properties. Moreover, because aliphatic or alicyclic polyisocyanates are expensive and the higher molecular weights of relatively inexpensive polyhydric alcohols per equivalent thereof renders the resulting coating composition less expensive, inexpensive polyhydric alcohols with the highest possible molecular weight have found wide acceptance.
Consequently, polyhydric alcohols having a high viscosity (above about 1000 centipoises) have been exclusively used in the past. On the other hand, known polyisocyanates have a viscosity of at least about 8000 centipoises at 25.degree. C in the absence of a solvent. Since the viscosity of the polyisocyanates must be reduced to about 300 centipoises or less, if possible to about 50 centipoises or less, in order to render the polyisocyanates coatable as a coating agent, the addition of a volatile low-viscosity solvent which does not react with the isocyanates has been considered essential in order to reduce their viscosity.
Polyisocyanates react with high-molecular-weight polyhydric alcohols, but their solubility in each other is not necessarily good. Accordingly, in order to obtain a homogeneous reaction product, in the past the addition of a solvent has been considered essential. Thus, when conventional polyurethane coating compositions are coated, the practice is to incorporate a solvent, usually in an amount exceeding the equivalent weight, into the composition.
The solvent, however, involves the danger of a fire, is toxic to humans, and causes a serious pollution problem at the time of its volatilization. It is also known that, although aliphatic or alicyclic polyisocyanates have superior properties such as weatherability or non-yellowing property, they exhibit a very slow rate of reaction as compared with aromatic polyisocyanates, and about two days are required for aliphatic or alicyclic polyisocyanates to solidify at room temperature (e.g., about 20.degree.-30.degree. C), and therefore, that these coating agents tend to be difficult to handle.
Japanese Patent Application (OPI) No. 103624/73 discloses a solventless urethane coating composition comprising a liquid urethane prepolymer containing 0.90 to 1.50 equivalents, based on the hydroxyl group, of at least 3 terminal isocyanate groups, 50 to 90 mol% of a polyhydric alcohol containing at least 2 hydroxyl groups and 10 to 50 mol% of a high boiling alcohol containing one hydroxyl group.
The viscosity of a coating agent is desirably about 15 to 65 centipoises for spray coating, at most about 300 centipoises, preferably 30 to 85 centipoises, for electrostatic coating, and about 100 to 150 centipoises for brush coating. Thus, taking these coating methods into consideration, the preferred upper limit of the viscosity is about 300 centipoises. It has been found, however, that if no solvent is used, or if the amount of solvent is less than 20% by weight, sufficiently feasible polyurethane coating compositions with such a low viscosity cannot be obtained using conventional techniques.
It has been found that in order to obtain a coatable urethane composition having a viscosity of not more than about 300 centipoises at 25.degree. C even in the absence of a volatile organic solvent which does not react with an isocyanate, a monohydric alcohol is incorporated in a larger amount than in the conventional compositions thereby to reduce the viscosity of the composition, and a low-molecular-weight polyhydric alcohol is incorporated thereby to form cross-links after reaction and to provide coatings with good properties.